Vertical light emitting diodes typically employ metal substrates and top contact regions. These characteristically introduce optical loss to trapped waveguide modes residing in the light emitting region. Currently, top surface roughening techniques are employed to extract the maximum amount of light on the first pass of light incident on the top surface. However, if light is multiply scattered by the bottom metal reflector then loss is introduced to the trapped waveguide mode.
Light emitting diodes (LEDs) are based on a forward biased p-n junction. Recently, LEDs have reached high brightness levels that have allowed them to enter into new solid state lighting applications as well as replacements for high brightness light sources such as light engines for projectors and automotive car headlights. These markets have also been enabled by the economical gains achieved through the high efficiencies of LEDs, as well as reliability, long lifetime and environmental benefits. These gains have been partly achieved by use of LEDs that are capable of being driven at high currents and hence produce high luminous outputs while still maintaining high wall plug efficiencies.
Solid state lighting applications require that LEDs exceed efficiencies currently achievable by alternative fluorescent lighting technologies. The efficiencies of LEDs can be quantified by three main factors, namely internal quantum efficiency, injection efficiency, and extraction efficiency. The latter being the basis for the present invention.
One of the main limiting factors reducing the extraction efficiency in LEDs is the emitted photons being totally internally reflected and trapped in the high refractive index of the epi-material. These trapped waveguide modes propagate in the LED structure until they are scattered, escape or reabsorbed. The thickness of the LED structure determines the number of modes that can be set up.
Many methods have been successfully employed to improve light extraction in LED heterostructures. These include shaping LED die, as described in U.S. Pat. Nos. 6,015,719 and 6,323,063, flip-chip mounting of LEDs as described by Wierer et al. in Appl. Phys. Lett., 78, Pg. 3379, 2001, roughening of the top surface as taught by Schnitzer et al in Applied Physics Letters 63, 2174, 1993, and using omnidirectional reflectors as suggested by Fink et al. in Science vol. 282, Pg. 1679, 1998. Other suggested methods include the use of periodic texturing on at least one interface of the structure to improve light extraction out of the light emitting region, as suggested in U.S. Pat. No. 5,779,924.
To provide light emitting devices with high current and thermal driving capabilities the vertical type n-p contact configuration in GaN material systems has been recently adopted. Such examples have been disclosed in U.S. Pat. No. 6,884,646 and U.S. Pat. 20060154389A1. However, one major drawback with such vertical type light emitting structures is the existence of optically lossy metal contacts in the close vicinity of the light emitting heterostructure. Trapped modes in the high index light emitting device typically undergo multiple internal reflections. The photons reflected at the interface between the metallic contact surface and the heterostructure material experiences large losses and hence reduces the total light output of the light emitting diode.
In U.S. Pat. No. 6,784,462 the use of an omni-directional reflector is proposed. This single dielectric electrically insulating layer is disposed between the light emitting region and the lower conductive region and having a plurality of electrical conductive vias contacting the lower light emitting region and an electrical contact. It is typically an object of vertical light emitting devices to provide good electrical and thermal conduction. However, the single dielectric layer residing between the light emitting region and the lower conductive region hinders good electrical conduction. In addition, a single dielectric layer will not provide true omni-directional reflectivity and light at angles residing within the escape cone formed between the light emitting medium and the dielectric layer will experience a reflection at the metal contact boundary, which will introduce optical loss. Additional loss in the light emitting device will also be experienced by the metal electrical contacts at the top surface of the device, which is not desirable.
Back Light Units (BLU) for LCD panels are key elements in the performance of an LCD panel. Currently most LCD panels employ compact cathode fluorescent light (ccfl) sources. However, these suffer from several problems such as poor colour gamut, environmental recycling and manufacture issues, thickness and profile, high voltage requirements, poor thermal management, weight and high power consumption. In order to alleviate these problems LCD manufacturers are implementing LED BLU units. These offer benefits in improved light coupling, colour gamut, lower power consumption, thin profiles, low voltage requirements, good thermal management and low weight.
Another application area for the present invention is in light engines for front and rear projectors. Conventional High Intensity Discharge (HID) type projector light engines have always been hindered by low efficiency and short lifetime resulting in slow adoption into consumer markets.
The present invention is directed towards another technique for improving the efficiency of LEDs, thereby enabling their use in various applications, including those described above.